This invention relates to the field of electro-optical energy conversion apparatus and is primarily concerned with "mosaic" detector arrays which provide surveillance of an extensive scene. The mosaic detector arrays are large numbers of closely spaced individual photo-detector elements arranged in essentially a two-dimensional, or planar, array. Broadly, the invention concerns an apparatus and method relating to a module which contains densely-packed electronic components and which is adapted to carry on a planar supporting surface an array of closely-spaced but isolated electro-optical elements which cause radiative/electronic energy conversion.
An earlier patent application of Carson and Dahlgren, Ser. No. 855,242, filed Nov. 28, 1977, issued as U.S. Pat. No. 4,304,624 on Dec. 8, 1981 and assigned to the assignee of this application, discloses a detector module comprising a number of thin insulating (ceramic) layers, or substrates, secured together and extending at right angles to the focal plane of the detector array. Electrically conducting leads are supported by and located between such layers for the purpose of conducting signals away from the detectors located in the focal plane. The prefabricated layered supporting structure thus constitutes the substrate for the detector array, which is located on one end (the focal plane) of the layered supporting structure. The individual detector elements located on the focal plane are each in separate electrical contact with one of the leads appearing at the adjacent edges of the layers which constitute the supporting structure.
A large number of the separately fabricated layered structures, or modules, are butted together to form an extensive focal plane array, i.e., the individual modules are used as building blocks to build up a complete focal plane array. The completed assembled array should be as light and as small as possible, in order to facilitate its use in such fields as satellite surveillance. At the same time, it is necessary to accomplish very significant amounts of electronic processing within the focal plane array, which means that each module, or layered structure, in the array must incorporate in its "body portion" a substantial amount of electronic processing capability. Otherwise, it would not be feasible to provide electrical connections between the detector array and the processing equipment remote from the focal plane.
As pointed out in U.S. Pat. No. 4,304,624, in order that the detector-carrying modules may be butted against one another to provide a continuous surveillance array, it is necessary that any electronics contained within the modules be housed in the "volume" of space created by extending the two-dimensional surface area of the focal plane detector array in a direction perpendicular to that two-dimensional surface. Nothing must protrude from the "sides" of the individual modules; and their electrical connections to the remotely located processing equipment should, if possible, be formed by leads on the back planes of the modules.
Some of the primary considerations in developing improved modules for focal plane detector arrays are:
(1) Optimizing packing density to achieve the lowest possible mass per element, essentially the shortest module that is production worthy;
(2) Assuring end-to-end continuity of the electrical leads;
(3) Simplifying the fabrication procedures;
(4) Enhancing the structural strength of the elements;
(5) Improving reliability of the electronics and of the techniques for interconnecting the electronics with the electrical leads;
(6) Maximizing suitability for test and repair at the lowest level of assembly and throughout assembly; and
(7) Minimizing the cost per element.
It is the general purpose of the present invention to make a major improvement in the apparatus and method designed to attain the listed objectives. One of the major limitations encountered with the ceramic-substrate layered modules of U.S. Pat. No. 4,304,624 has been the inefficient use of "real estate" because the lead patterns consume most of the available space. Other advantageous aspects of the present invention over the prior design discussed above are the elimination of the multiplicity of wire bonds in the module electronics, and the utilization (in the present invention) of the rear plane of the module as a surface for printed circuitry which provides electrical connection to the remote signal processing equipment.
The prior art includes Lehovec U.S. Pat. No. 3,748,479, issued July 24, 1973, and its parent U.S. Pat. No. 3,631,251, issued Dec. 28, 1971. Those patents disclose the use of a stack of silicon slabs having p-n junction photocells on their narrow edges to provide a focal plane surface, and having electrical circuit elements, including transistors, formed on the larger planar surfaces of silicon material. The Lehovec patent applies the concept of stacked silicon slabs to light emission, reception, or modulation. In the light-reception versions, which are of interest in connection with the present invention, the light sensors, or photo-detectors, in Lehovec are integrally formed as p-n junction photocells in the silicon material.
A major shortcoming of the Lehovec light-reception structures is the reliance on the treated silicon material for light sensing. This limits significantly the effective light-sensing capability of the Lehovec device. The detector material does not respond adequately unless cooled to extremely low temperatures, lower than those required for other detector materials, such as may be used with structures described in this disclosure. Furthermore, the present arts in silicon detector processing and in silicon electronics processing are not fully compatible with one another, in that high performance cannot be simultaneously achieved in detectors and electronics.
As disclosed in Rotolante and Koehler application Ser. No. 15,070, filed Feb. 26, 1979, issued as U.S. Pat. No. 4,290,844 on Sept. 22, 1981, and assigned to a company related to the assignee of this application, effective photo-detector arrays on modules of the type under discussion require the use of specialized detector materials, such as mercury-cadmium-telluride, indium antimonide, lead tin telluride, indium arsenic antimonide, indium arsenide, and indium gallium arsenide. And such detector materials necessitate a "hybrid" technology for securing the detectors to the focal plane provided by the narrow ends of the stacked substrates.
The present invention is intended to solve the problems which must be overcome in order to combine the benefits of the "hybrid" detector concepts with the use of stacked silicon chips which carry integrated electronic circuits, and which are efficiently connected to external circuitry.